1. Field of the Invention
The present invention generally relates to an electroluminescence device and a flat panel display using an electroluminescence device, and more particularly to an organic electroluminescence device and an organic electroluminescence display.
2. Description of the Related Art
In recent years, demands of the market are shifting from a conventional large size and heavy-weight CRT (Braun tube) display to a thin-size and light-weight flat display. As for the flat displays, there are liquid crystal displays and plasma displays which are employed, for example, as home television receivers and monitors of personal computers.
Currently, an electroluminescence display (hereinafter referred to as “EL display”) and more particularly, an organic electroluminescence display is drawing attention as the next generation flat display. Ever since the report on a multilayered type element being layered with organic thin films having a hole transport property and an electron transport property (C. W.Tang and S. A. Van Slyke, Applied Physics Letters vol. 51, 913 (1987), organic EL devices included in the organic EL display are gathering attention as a large area light emitting element that illuminates with a low voltage of 10 V or less, and are being researched extensively. Compared to the liquid crystal display, the organic EL display requires no backlight since the organic EL display is self-luminous, thereby yielding a thinner, simple-structured, and flexible display. Therefore, the organic EL display is expected to be widely applied in various fields. In employing the organic EL display for practical use, however, a task of obtaining a longer life-span for the organic EL display is yet to be achieved.
FIG. 1 is a cross-sectional drawing showing a conventional organic EL device. As shown in FIG. 1, an organic EL device 10 has a transparent anode 12, a hole injection layer 13, a hole transport layer 14, a luminescent layer 15, an electron transport layer 16, and a cathode 18 which are formed in this order on a transparent insulating substrate 11. In the organic EL device 10, holes are injected from the transparent anode 12 to the hole injection layer 13, while electrons are injected from the cathode 18. The holes and electrons are recombined in the luminescent layer 15, to thereby release energy. This energy excites, for example, an organic fluorescent part in the luminescent layer 15, thereby causing illumination. Luminance is determined by the amount of recombination of the recombining holes and electrons with respect to time. Since luminous efficiency is expressed by luminance with respect to current consumption, the luminous efficiency becomes higher when the amount of electrons and holes contributing to illumination is more balanced.
In the organic EL device 10, the transparent anode 12 is formed of ITO (Indium Tin Oxide). By oxidizing the surface of the ITO with, for example, UV ozone or oxygen plasma, the work function can be consistent with the ionization potential of the hole injection layer. This reduces the hole injection barrier between the transparent anode 12 to the hole injection layer 13 and increases the amount of hole current.
Meanwhile, metals, such as Li, Mg, or alloys such as Al—Li, Mg—Ag, which have a low electron injection barrier work function with respect to the electron transport layer 16, are used in the cathode 18. In recent years, it is found that use of a metal fluoride (e.g. LiF/Al) for the electron injection layer, even in a case of employing a simple Al as the cathode 18, yields an electron injection performance that is equal to a device using low work function simple metals, such as Li, Mg or alloys thereof, and that device properties, such as luminous efficiency, are the same as or greater than using metals with low work function (L. S. Hung, C. W. Tang Tang, and M. G. Mason, Applied Physics Letters vol. 70 (2), 152 (1997).
Nevertheless, when a low work function simple metal, alloys or LiF, for example, is used in the cathode 18, the amount of electron current reaching the luminescent layer 15 becomes less than the amount of hole current, thereby causing unevenness in the amount of current between the electrons and the holes. This causes the current of the holes not contributing to illumination to become wasted and prevents a sufficient luminous efficiency from being obtained.
Furthermore, in a case where luminous efficiency is low, the amount of voltage is to be increased for obtaining a sufficient luminance and supplying a greater amount of current. However, an excess application of voltage is liable to cause a chemical reaction at interfaces between the anode 12 and the hole injection layer 13 and between the cathode 18 and the electron transport layer 16. This leads to property change and function deterioration in the hole injection layer 13 and the electron transport layer 16, and furthermore, device failure. Therefore, there is a problem in obtaining a sufficient life-span for the device.
Furthermore, Japanese Laid-Open Patent Application No. 2002-43063 discloses a multilayer electron transport area provided for improving carrier injection to a luminescent layer and reducing operating voltage. However, this document does not disclose a specific configuration of the structure of the multilayer electron transport area.